1. Field of the Invention
The present invention relates to an output voltage detection circuit for a traffic signal controller, and in particular to an improved output voltage detection circuit for a traffic signal controller which is capable of reducing a characteristic variation based on an operational temperature variation, implementing a hybrid integration device, enabling a compact product, and adjusting a detection reference point by a software-based method.
2. Description of the Conventional Art
In the traffic signal controller, there is provided a function that the state of a traffic light ON/OFF signal (lamp on/off signal) is detected in real time for evaluating whether a signal control is normally performed.
The method for detecting whether a signal control is normally performed is classified into two methods. Between the methods, a method is directed to detecting whether a voltage is normally supplied to a traffic light ON/OFF cable which outputs an ON signal, and the other method is directed to detecting whether a voltage is not supplied to a traffic light ON/OFF cable which outputs an OFF signal.
Generally, since checking whether an ON state is correctly generated is important, among the above-described detection methods, the previous method is used for the group of red lights (PR: a pedestrian red light, R: a vehicle red light, Y: a vehicle yellow light). In addition, since whether an OFF state is correctly generated is important, among the above-described detection methods, the later method is used for the group of green light (PG: a pedestrian green light, G: a vehicle green light, A: a vehicle left-turn or right-turn light.
In the traffic signal controller, to detect whether a voltage is normally inputted into the traffic signal controller is important. In the normal state, a traffic signal control is continuously performed. In the abnormal state, a traffic signal control is stopped because a traffic accident may occur, and an on-and-off operation is performed.
As shown in FIG. 1, the conventional output voltage detection circuit for a traffic signal controller includes a detection voltage dividing unit 11 for dividing a detection input voltage VI having an alternating current component, which is applied to a traffic signal system, into voltages having a predetermined level, a photo-coupler 12 for receiving an output voltage from the detection voltage dividing unit 11, and a signal converter 13 for outputting a low level voltage when a voltage is periodically inputted through the photo-coupler 12 and outputting a high level voltage when a voltage is periodically not inputted.
Here, the detection voltage dividing unit 11 includes a resistor R11 which is connected in series with the input terminal, and a diode D11 and a resistor R12 which are connected in parallel with the resistor R11 and the input terminal.
The photo-coupler 12 includes a light emitting diode LED connected in parallel with the detection voltage dividing unit 11 and the resistor R12, and a photo-transistor PTR which is turned on and off in accordance with a detection of the light emitting diode LED.
In addition, the signal converter 13 includes a resistor R13 connected between a power terminal voltage VCC and a photo-transistor PTR of the photo-coupler 12, a resistor R14 and a condenser C11 connected in series between the collector of he photo-transistor PTR of the photo-coupler 12 and a ground, a diode D12 connected in parallel with the resistor R14, and an output buffer BUF11 commonly connected with the resistor R14 and the condenser C11 for outputting an output detection voltage VO.
The operation of the conventional output voltage detection circuit for a traffic signal controller will now be explained with reference to the accompanying drawings.
First, when a power is supplied to a traffic signal ON/OFF cable for turning on/off the traffic signals, the detection input voltage VI having an alternating current component which is fed-back from the traffic signal ON/OFF cable is divided into voltages having a predetermined level by the resistors R11 and R12 and the thusly divided voltages are supplied to the photo-coupler 12.
Here, when the detection input voltage VI is normally inputted, the level thereof is ranged from 120 volts to 220 volts. The light emitting diode LED and the photo-transistor PTR of the photo-coupler 12 are repeatedly turned on/off at a period corresponding to the power frequency.
Namely, when the detection input voltage VI is inputted, the photo-transistor PTR of the photo-coupler 12 is turned on, and the power terminal voltage VCC is applied to the ground terminal through the photo-transistor PTR. A waveform as shown in FIG. 2B is outputted in the interval T1 from the collector of the photo-transistor PTR. When the detection input voltage VI is not applied, since the photo-transistor PTR of the photo-coupler 12 is turned off, and the power terminal voltage VCC is charged by a time constant (T=R14.times.C11) into the condenser C11 through the resistor R14, the collector of the photo-transistor PTR outputs a high level voltage in the interval T2 as shown in FIG. 2B.
At this time, at the moment when the photo-transistor PTR is turned on, since the voltage is quickly discharged from the diode D12 and the photo-transistor PTR of the signal converter 13 to a ground, while the power is supplied to the traffic signal on/off cable, namely, while the detection input voltage VI is inputted, the output buffer BUF11 outputs a low level detection output voltage VO in the interval T1 as shown in FIG. 2.
On the contrary, when a detection input voltage VI is not inputted, a high level detection output voltage VO as shown in FIG. 2C is outputted in the interval T2.
The detection output voltage VO outputted from the output buffer BUF11 of the signal converter 13 is applied to the system controller (not shown). The system controller (not shown) judges whether a voltage input/output is normally performed with respect to the traffic signal.
Generally, the operational temperature of the traffic signal system is set at -34.degree. C..about.+74.degree. C. The voltage feeding-back system is configured such that an input voltage is not detected below AC 15 volts, and the input voltage is detected above AC 25 volts.
In addition, the traffic signal controller is configured such that 48 traffic lights are driven. Therefore, the traffic signal controller outputs 48 has an output circuit of 48 signals. Therefore, in the output voltage feeding-back circuit, the routines are performed 48 time.
However, in the conventional output voltage detection circuit for a traffic signal controller, the photo-coupler 12 is installed as an important element in the circuit. Since the turning-on current of the photo-coupler 12 is varied, the start point of the detection voltage is disadvantageously varied. In addition, it is difficult to configure the apparatus which is capable of satisfying the voltage detection characteristic within a range of the operational temperature of the apparatus.
Furthermore, since it is difficult to manufacture a hybrid IC consisting of the photo-transistor PTR and the light emitting device (LED), it is impossible to manufacture a compact product.